Read up on the Glycemic Index. Lots of weigh gain associated with insulin regulation. All carbs are not created equal. The trick is to pick the right ones that are slow to metabolized (and processed foods are not). That keeps your body in fat burning mode instead of blood sugar burning mode (storing fat). It's very important for avoiding Type 2 diabetes as well.
Protein, fiber, fat and acid are things that slow the carb metabolizing rate, and help regulate insulin spikes. Again, the trick is to find good sources. For example, bacon vs nuts for fat intake would seem like a no brainer, but a lot of carb reduction diets don't care where the fat comes from, and you still see the weight reduction results........But that comes with it's own potential problems.
Good luck
